Microbubble Compositions, Properties and Biomedical Applications

Enhancing In Vitro Stability of Albumin Microbubbles Produced Using Microfluidic T-Junction Device

Microfluidics is an efficient technique for continuous synthesis of monodispersed microbubbles. However, microbubbles produced using microfluidic devices possess lower stability due to quick dissolution of core gas when exposed to an aqueous environment. This work aims at generating highly stable monodispersed albumin microbubbles using microfluidic T-junction devices. Microbubble generation was facilitated by an aqueous phase consisting of bovine serum albumin (BSA) as a model protein and nitrogen (N₂) gas. Microbubbles were chemically cross-linked using dilute glutaraldehyde (0.75% v/v) solution and thermally cross-linked by collecting microbubbles in hot water maintained at 368 (±2) K. These microbubbles were then subjected to in vitro dissolution in an air-saturated water. Microbubbles cross-linked with a combined treatment of thermal and chemical cross-linking (TC & CC) had longer dissolution time compared to microbubbles chemically cross-linked (CC) alone, thermally cross-linked (TC) alone, and non-cross-linked microbubbles. Circular dichroism (CD) spectroscopy analysis revealed that percent reduction in alpha-helices of BSA was higher for the combined treatment of TC & CC when compared to other treatments. In contrast to non-cross-linked microbubbles where microbubble shell dissolved completely, a significant shell detachment was observed during the final phase of the dissolution for cross-linked microbubbles captured using high speed camera, depending upon the extent of cross-linking of the microbubble shell. SEM micrographs of the microbubble shell revealed the shell thickness of microbubbles treated with TC & CC to be highest compared to only thermally or only chemically cross-linked microbubbles. Comparison of microbubble dissolution data to a mass transfer model showed that shell resistance to gas permeation was highest for microbubbles subjected to a combined treatment of TC & CC.

Ultrasound-induced biophysical effects in controlled drug delivery

Ultrasound is widely used in biomedical engineering and has applications in conventional diagnosis and drug delivery. Recent advances in ultrasound-induced drug delivery have been summarized previously in several reviews that have primarily focused on the fabrication of drug delivery carriers. This review discusses the mechanisms underlying ultrasound-induced drug delivery and factors affecting delivery efficiency, including the characteristics of drug delivery carriers and ultrasound parameters. Firstly, biophysical effects induced by ultrasound, namely thermal effects, cavitation effects, and acoustic radiation forces, are illustrated. Secondly, the use of these biophysical effects to enhance drug delivery by affecting drug carriers and corresponding tissues is clarified in detail. Thirdly, recent advances in ultrasound-triggered drug delivery are detailed. Safety issues and optimization strategies to improve therapeutic outcomes and reduce side effects are summarized. Finally, current progress and future directions are discussed.

Application of Ultrasound Combined with Microbubbles for Cancer Therapy

At present, cancer is one of the leading causes of death worldwide. Treatment failure remains one of the prime hurdles in cancer treatment due to the metastatic nature of cancer. Techniques have been developed to hinder the growth of tumours or at least to stop the metastasis process. In recent years, ultrasound therapy combined with microbubbles has gained immense success in cancer treatment. Ultrasound-stimulated microbubbles (USMB) combined with other cancer treatments including radiation therapy, chemotherapy or immunotherapy has demonstrated potential improved outcomes in various in vitro and in vivo studies. Studies have shown that low dose radiation administered with USMB can have similar effects as high dose radiation therapy. In addition, the use of USMB in conjunction with radiotherapy or chemotherapy can minimize the toxicity of high dose radiation or chemotherapeutic drugs, respectively. In this review, we discuss the biophysical properties of USMB treatment and its applicability in cancer therapy. In particular, we highlight important preclinical and early clinical findings that demonstrate the antitumour effect combining USMB and other cancer treatment modalities (radiotherapy and chemotherapy). Our review mainly focuses on the tumour vascular effects mediated by USMB and these cancer therapies. We also discuss several current limitations, in addition to ongoing and future efforts for applying USMB in cancer treatment.

Preparation of Nanoparticle-Loaded Microbubbles via an Electrohydrodynamic Atomization Process

Microbubbles have been widely used in many research fields due to their outstanding physicochemical properties and unique structural characteristics, especially as ultrasonic contrast agents and drug delivery carriers. However, the stability of conventional microbubbles is generally poor, which limits the development of their applications. Loading nanoparticle to microbubbles has great potential in enhancing the stability of microbubbles. This paper reports for the first time the feasibility of one-step preparation of nanoparticle-loaded microbubbles by coaxial electrohydrodynamic atomization. Bovine serum albumin (BSA) was used as the model material of the bubble shell layer to study the effect of the loading of nanoparticles on the stability of microbubbles. The results show that the concentration of nanoparticles has a significant impact on the stability of microbubbles, and loading an appropriate amount of nanoparticles is helpful in improving the stability of microbubbles. The results also show that nanoparticle-loaded microbubbles with a size distribution in the range of 120–200 μm can be prepared under optimal conditions.

Advances in the Sensing and Treatment of Wound Biofilms

Wound biofilms represent a particularly challenging problem in modern medicine. They are increasingly antibiotic resistant and can prevent the healing of chronic wounds. However, current treatment and diagnostic options are hampered by the complexity of the biofilm environment. In this review, we present new chemical avenues in biofilm sensors and new materials to treat wound biofilms, offering promise for better detection, chemical specificity, and biocompatibility. We briefly discuss existing methods for biofilm detection and focus on novel, sensor-based approaches that show promise for early, accurate detection of biofilm formation on wound sites and that can be translated to point-of-care settings. We then discuss technologies inspired by new materials for efficient biofilm eradication. We focus on ultrasound-induced microbubbles and nanomaterials that can both penetrate the biofilm and simultaneously carry active antimicrobials and discuss the benefits of those approaches in comparison to conventional methods.

Advances in the Sensing and Treatment of Wound Biofilms

Wound biofilms represent a particularly challenging problem in modern medicine. They are increasingly antibiotic resistant and can prevent the healing of chronic wounds. However, current treatment and diagnostic options are hampered by the complexity of the biofilm environment. In this review, we present new chemical avenues in biofilm sensors and new materials to treat wound biofilms, offering promise for better detection, chemical specificity, and biocompatibility. We briefly discuss existing methods for biofilm detection and focus on novel, sensor-based approaches that show promise for early, accurate detection of biofilm formation on wound sites and that can be translated to point-of-care settings. We then discuss technologies inspired by new materials for efficient biofilm eradication. We focus on ultrasound-induced microbubbles and nanomaterials that can both penetrate the biofilm and simultaneously carry active antimicrobials and discuss the benefits of those approaches in comparison to conventional methods.

Ultrasound-Responsive Systems as Components for Smart Materials

Smart materials can respond to stimuli and adapt their responses based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications such as actuation, sensing, or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of time scales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering, and acoustic radiation forces to enable capabilities such as actuation, sensing, payload delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound and poses interesting questions for further investigations.

Investigation of the Phase of Nonlinear Echoes From Microbubbles During Amplitude Modulation

Contrast-enhanced ultrasound (CEUS) imaging relies on distinguishing between microbubble and tissue echoes. Amplitude modulation (AM), a nonlinear pulsing scheme, has been developed to take advantage of the amplitude-dependent nonlinearity of microbubble echoes. However, with AM, tissue nonlinear propagation can also degrade the image contrast. Segmentation of CEUS images based on amplitude-dependent phase difference in the echoes, defined in this article as [Formula: see text], has been proposed as an additional method of enhancing contrast-to-tissue ratio as tissue is not expected to create the same degree of [Formula: see text]; however, this has not been robustly investigated. In this work, we evaluate the source of [Formula: see text] through simulations of unshelled versus shelled microbubble oscillation and simulations of nonlinear propagation in tissue. We then validate the simulated [Formula: see text] results with experimental [Formula: see text] measurements during in vitro scattering and imaging in a flow phantom. We show that shelled and unshelled microbubbles resulted in a [Formula: see text] with similar overall magnitude with some differences in trends, and that tissue echoes have a small yet detectable degree of [Formula: see text] due to nonlinear propagation. The results from this work can help inform optimal parameter selection for phase segmentation and implementation on a clinical scanner.

Engineering Responsive Ultrasound Contrast Agents Through Crosslinked Networks on Lipid-Shelled Microbubbles

Ultrasound imaging with contrast agents, especially with lipid-shelled microbubbles, has become a vital tool in clinical diagnostics. Efforts to adapt these agents for molecular imaging have typically focused on targeted binding. More recently, crosslinking the lipid shell to alter its mechanical properties, followed by decrosslinking upon exposure to a stimulus, has been shown as a promising approach for imaging soluble molecular targets. Nevertheless, a systematic study of the influence of crosslinker concentration and structure on the mechanical properties of microbubbles has not been undertaken. An improved understanding of the role of these parameters is necessary to more effectively design contrast agents that detect proteases, an informative class of soluble disease markers. Here, the influence of crosslinker parameters on the acoustic properties of microbubbles, developing a model of crosslinker network formation on microbubble shells that explains the experimental observations, are studied. By incorporating cleavable elements that respond to UV light or proteolysis, kinetically resolved acoustic detection of these stimuli and the relevance of crosslinker design are demonstrated. The framework established in this study can be readily adapted to other protease-cleavable units and provides a basis for the future development of responsive ultrasound contrast agents for molecular imaging of proteolytic activity.

Protein-conjugated microbubbles for the selective targeting of S. aureus biofilms

Staphylococcus aureus (S. aureus) is an important human pathogen and a common cause of bloodstream infection. The ability of S. aureus to form biofilms, particularly on medical devices, makes treatment difficult, as does its tendency to spread within the body and cause secondary foci of infection. Prolonged courses of intravenous antimicrobial treatment are usually required for serious S. aureus infections. This work investigates the in vitro attachment of microbubbles to S. aureus biofilms via a novel Affimer protein, AClfA1, which targets the clumping factor A (ClfA) virulence factor – a cell-wall anchored protein associated with surface attachment. Microbubbles (MBs) are micron-sized gas-filled bubbles encapsulated by a lipid, polymer, or protein monolayer or other surfactant-based material. Affimers are small (∼12 kDa) heat-stable binding proteins developed as replacements for antibodies. The binding kinetics of AClfA1 against S. aureus ClfA showed strong binding affinity (K_D = 62 ± 3 nM). AClfA1 was then shown to bind S. aureus biofilms under flow conditions both as a free ligand and when bound to microparticles (polymer beads or microbubbles). Microbubbles functionalized with AClfA1 demonstrated an 8-fold increase in binding compared to microbubbles functionalized with an identical Affimer scaffold but lacking the recognition groups. Bound MBs were able to withstand flow rates of 250 μL/min. Finally, ultrasound was applied to burst the biofilm bound MBs to determine whether this would lead to biofilm biomass loss or cell death. Application of a 2.25 MHz ultrasound profile (with a peak negative pressure of 0.8 MPa and consisting of a 22-cycle sine wave, at a pulse repetition rate of 10 kHz) for 2 s to a biofilm decorated with targeted MBs, led to a 25% increase in biomass loss and a concomitant 8% increase in dead cell count. The results of this work show that Affimers can be developed to target S. aureus biofilms and that such Affimers can be attached to contrast agents such as microbubbles or polymer beads and offer potential, with some optimization, for drug-free biofilm treatment.

Phase-shift nanodroplets as an emerging sonoresponsive nanomaterial for imaging and drug delivery applications

Nanodroplets - emerging phase-changing sonoresponsive materials - have attracted substantial attention in biomedical applications for both tumour imaging and therapeutic purposes due to their unique response to ultrasound. As ultrasound is applied at different frequencies and powers, nanodroplets have been shown to cavitate by the process of acoustic droplet vapourisation (ADV), causing the development of mechanical forces which promote sonoporation through cellular membranes. This allows drugs to be delivered efficiently into deeper tissues where tumours are located. Recent reviews on nanodroplets are mostly focused on the mechanism of cavitation and their applications in biomedical fields. However, the chemistry of the nanodroplet components has not been discussed or reviewed yet. In this review, the commonly used materials and preparation methods of nanodroplets are summarised. More importantly, this review provides examples of variable chemistry components in nanodroplets which link them to their efficiency as ultrasound-multimodal imaging agents to image and monitor drug delivery. Finally, the drawbacks of current research, future development, and future direction of nanodroplets are discussed.

Mechanistic Insights and Therapeutic Delivery through Micro/Nanobubble-Assisted Ultrasound

Ultrasound with low frequency (20–100 kHz) assisted drug delivery has been widely investigated as a non-invasive method to enhance the permeability and retention effect of drugs. The functional micro/nanobubble loaded with drugs could provide an unprecedented opportunity for targeted delivery. Then, ultrasound with higher intensity would locally burst bubbles and release agents, thus avoiding side effects associated with systemic administration. Furthermore, ultrasound-mediated destruction of micro/nanobubbles can effectively increase the permeability of vascular membranes and cell membranes, thereby not only increasing the distribution concentration of drugs in the interstitial space of target tissues but also promoting the penetration of drugs through cell membranes into the cytoplasm. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theragnostic tool. In this review, we first discuss the structure and generation of micro/nanobubbles. Second, ultrasound parameters and mechanisms of therapeutic delivery are discussed. Third, potential biomedical applications of micro/nanobubble-assisted ultrasound are summarized. Finally, we discuss the challenges and future directions of ultrasound combined with micro/nanobubbles.

A comprehensive update of micro- and nanobubbles as theranostics in oncology

Advances in diagnostic and imaging capabilities have allowed cancers to be detected earlier and characterized more robustly. These strategies have recently branched into theranostics whereby contrast agents traditionally used for imaging have been co-loaded with therapeutics to simultaneously diagnose and treat cancers in a patient-specific manner. Microbubbles (MB) and nanobubbles (NB) are contrast agents which can be modulated to meet the theranostic needs particularly in the realm of oncology. The current review focuses on the ultrasound-responsive MB/NB platforms used as a theranostic tool in oncology. We discuss in detail the key parameters that influence the utility of MB/NB formulations and implications of such treatment modalities. Recent advances in composition strategies, latest works in the pre-clinical stages and multiple paradigm-shifting innovations in the field of MB/NB are discussed in-depth in this review. The clinical application of MB/NB is currently limited to diagnostic imaging. Surface chemistry modification strategies will help tune the formulations toward therapeutic applications. It is also anticipated that MB/NB will see increased use to deliver gas therapeutics. Scalability and stability considerations will be at the forefront as these particles get introduced into the clinical theranostic toolbox.

Developing a Microbubble-Based Contrast Agent for Synchrotron Multiple-Image Radiography

PURPOSE

Multiple-image radiography (MIR) is an analyzer-based synchrotron X-ray imaging approach capable of dissociating absorption, refraction, and scattering components of X-ray interaction with the material. It generates additional image contrast mechanisms (besides absorption), especially in the case of soft tissues, while minimizing absorbed radiation dose. Our goal is to develop a contrast agent for MIR using ultrasound microbubbles by carrying out a systematic assessment of size, shell material, and concentration.

PROCEDURES

Microbubbles were synthesized with two different shell materials: phospholipid and polyvinyl-alcohol. Polydisperse perfluorobutane-filled lipid microbubbles were divided into five size groups using centrifugation. Two distributions of air-filled polymer microbubbles were generated: 2-3 µm and 3-4 µm. A subset of polymer microbubbles 3-4 µm had iron oxide nanoparticles incorporated into their shell or coated on their surface. Microbubbles were immobilized in agar with different concentrations: 5 × 10⁷, 5 × 10⁶, and 5 × 10⁵ MBs/ml. MIR was conducted on the BioMedical Imaging and Therapy beamline at the Canadian Light Source. Three images were generated: Gaussian amplitude, refraction, and ultra-small-angle X-ray scattering (USAXS). The contrast signal was quantified by measuring mean pixel values and comparing them with agar.

RESULTS

No difference was detected in absorption or refraction images of all tested microbubbles. Using USAXS, a significant signal increase was observed with lipid microbubbles 6-10 µm at the highest concentration (p = 0.02), but no signal was observed at lower concentrations.

CONCLUSIONS

These data indicate that lipid microbubbles 6-10 µm are candidates as contrast agents for MIR, specifically for USAXS. A minimum concentration of 5 × 10⁷ microbubbles (lipid-shell 6-10 µm) per milliliter was needed to generate a detectable signal.

Identifying Unique Acoustic Signatures from Chemically-Crosslinked Microbubble Clusters Using Deep Learning

Ultrasound contrast agents (UCA) are gas encapsulated microspheres that oscillate volumetrically when exposed to an ultrasound field producing a backscattered signal which can be used for improved ultrasound imaging and drug delivery. UCA's are being used widely for contrast-enhanced ultrasound imaging, but there is a need for improved UCAs to develop faster and more accurate contrast agent detection algorithms. Recently, we introduced a new class of lipid based UCAs called Chemically Cross-linked Microbubble Clusters (CCMCs). CCMCs are formed by the physical tethering of individual lipid microbubbles into a larger aggregate cluster. The advantages of these novel CCMCs are their ability to fuse together when exposed to low intensity pulsed ultrasound (US), potentially generating unique acoustic signatures that can enable better contrast agent detection. In this study, our main objective is to demonstrate that the acoustic response of CCMCs is unique and distinct when compared to individual UCAs using deep learning algorithms. Acoustic characterization of CCMCs and individual bubbles was performed using a broadband hydrophone or a clinical transducer attached to a Verasonics Vantage 256. A simple artificial neural network (ANN) was trained and used to classify raw 1D RF ultrasound data as either from CCMC or non-tethered individual bubble populations of UCAs. The ANN was able to classify CCMCs at an accuracy of 93.8% for data collected from broadband hydrophone and 90% for data collected using Verasonics with a clinical transducer. The results obtained suggest the acoustic response of CCMCs is unique and has the potential to be used in developing a novel contrast agent detection technique.

Ultrasound-Mediated Drug Delivery: Sonoporation Mechanisms, Biophysics, and Critical Factors

Sonoporation, or the use of ultrasound in the presence of cavitation nuclei to induce plasma membrane perforation, is well considered as an emerging physical approach to facilitate the delivery of drugs and genes to living cells. Nevertheless, this emerging drug delivery paradigm has not yet reached widespread clinical use, because the efficiency of sonoporation is often deemed to be mediocre due to the lack of detailed understanding of the pertinent scientific mechanisms. Here, we summarize the current observational evidence available on the notion of sonoporation, and we discuss the prevailing understanding of the physical and biological processes related to sonoporation. To facilitate systematic understanding, we also present how the extent of sonoporation is dependent on a multitude of factors related to acoustic excitation parameters (ultrasound frequency, pressure, cavitation dose, exposure time), microbubble parameters (size, concentration, bubble-to-cell distance, shell composition), and cellular properties (cell type, cell cycle, biochemical contents). By adopting a science-backed approach to the realization of sonoporation, ultrasound-mediated drug delivery can be more controllably achieved to viably enhance drug uptake into living cells with high sonoporation efficiency. This drug delivery approach, when coupled with concurrent advances in ultrasound imaging, has potential to become an effective therapeutic paradigm.

Towards standardization of the parameters for opening the blood-brain barrier with focused ultrasound to treat glioblastoma multiforme: A systematic review of the devices, animal models, and therapeutic compounds used in rodent tumor models

Glioblastoma multiforme (GBM) is a deadly and aggressive malignant brain cancer that is highly resistant to treatments. A particular challenge of treatment is caused by the blood-brain barrier (BBB), the relatively impermeable vasculature of the brain. The BBB prevents large molecules from entering the brain parenchyma. This protective characteristic of the BBB, however, also limits the delivery of therapeutic drugs for the treatment of brain tumors. To address this limitation, focused ultrasound (FUS) has been safely utilized to create transient openings in the BBB, allowing various high molecular weight drugs access to the brain. We performed a systematic review summarizing current research on treatment of GBMs using FUS-mediated BBB openings in in vivo mouse and rat models. The studies gathered here highlight how the treatment paradigm can allow for increased brain and tumor perfusion of drugs including chemotherapeutics, immunotherapeutics, gene therapeutics, nanoparticles, and more. Given the promising results detailed here, the aim of this review is to detail the commonly used parameters for FUS to open the BBB in rodent GBM models.

Laboratory-Scale Production of Sterile Targeted Microbubbles

Perfluorocarbon gas-filled microbubbles are clinically used as ultrasound contrast agents. We have been developing targeted microbubbles based BACS (buoyancy activated cell sorting) or BUBLES (buoyancy enabled separation) for ex vivo cell isolation from bloods for circulating tumor cell (CTC) detection and hematopoietic cell isolation. Recently, we further applied targeted microbubbles for multimarker cell sorting, and as artificial antigen presenting cells (aAPC) for T cell activation and expansion by taking advantage of a number of interesting properties of lipid-shelled microbubbles. This chapter will describe the process of manufacturing sterile targeted microbubbles for research applications.

Subcutaneous Xenograft Models for Studying PDT In Vivo

The most facile, reproducible, and robust in vivo models for evaluating the anticancer efficacy of photodynamic therapy (PDT) are subcutaneous xenograft models of human tumors. The accessibility and practicality of light irradiation protocols for treating subcutaneous xenograft models also increase their value as relatively rapid tools to expedite the testing of novel photosensitizers, respective formulations, and treatment regimens for PDT. This chapter summarizes the methods used in the literature to prepare various types of subcutaneous xenograft models of human cancers and syngeneic models to explore the role of PDT in immuno-oncology. This chapter also summarizes the PDT treatment protocols tested on the subcutaneous models, and the procedures used to evaluate the efficacy at the molecular, macromolecular, and host organism levels.

Mesoporous Silica Nanoparticle-Based Imaging Agents for Hepatocellular Carcinoma Detection

Hepatocellular carcinoma (HCC) is characterized by poor prognosis and high mortality. The treatment of HCC is closely related to the stage, and the early-stage of HCC patients usually accompanies a more long-term survival rate after clinical treatment. Hence, there are critical needs to develop effective imaging agents with superior diagnostic precision for HCC detection at an early stage. Recently, mesoporous silica nanoparticles (MSNs) based imaging agents have gained extensive attentions in HCC detection, which can serve as a multifunctional nanoplatform with controllable size and facile surface functionalization. This perspective summarizes recent advances in MSNs based imaging agents for HCC detection by the incorporation of several clinical imaging modalities. Multi-modal imaging system has been developed for higher spatial resolution and sensitivity. Even though some limitations and challenges need to be overcome, we envision the development of novel MSNs based imaging agents will offer great potential applications in clinical HCC detection.

Theranostic nanobubbles towards smart nanomedicines

Targeted therapy and early accurate detection of malignant lesions are essential for the effectiveness of treatment and prognosis in cancer patients. The development of gaseous system as a versatile platform for the fabricated nanobubbles, has attracted much interest in improving the efficacy of ultrasound therapeutic, diagnostic, and theranostic platforms. Nano-sized bubble, as an ultrasound contrast agent, with spherical gas-filled structures exhibited contrast enhancement capability due to their inherent EPR effect. Additionally, nanobubbles exhibited good stability with extended retention time in the blood stream. The current review summarized various nanobubbles and discussed about the crucial parameters affecting the stability of ultrafine bubbles. Furthermore, therapeutic and theranostic gaseous systems for fighting against cancer were described.

Ultrasound contrast agents: microbubbles made simple for the pediatric radiologist

The ability to provide prompt, real-time, easily accessible and radiation-free diagnostic assessments makes ultrasound (US) one of the most versatile imaging modalities. The introduction and development of stable microbubble-based ultrasound contrast agents (UCAs) in the early 1990s improved visualization of complex vascular structures, overcoming some of the limitations of B-mode and Doppler imaging. UCAs have been used extensively in the adult population to visualize vasculature and to evaluate perfusion and blood flow dynamics in organs and lesions. Since the first observations that air bubbles within a liquid can generate a strong echogenic effect, to the early makeshift approaches with agitated saline, and later to the development of industrially produced and federally approved UCAs, these agents have evolved to become both clinically and commercially viable. Perhaps the most exciting potential of UCAs is being uncovered by current research that explores the use of these agents for molecular imaging and therapeutic applications. As contrast-enhanced ultrasound (CEUS) becomes more widely available, it is important for pediatric radiologists to understand the physics of the interaction between the US signal and the microbubbles in order to properly utilize them for the highest level of diagnostic imaging and interventions. In this article we introduce the composition of UCAs and the physics of their behavior in US, and we offer a brief history of their development over the last decades.

Applications of Ultrasound-Mediated Drug Delivery and Gene Therapy

Gene therapy has continuously evolved throughout the years since its first proposal to develop more specific and effective transfection, capable of treating a myriad of health conditions. Viral vectors are some of the most common and most efficient vehicles for gene transfer. However, the safe and effective delivery of gene therapy remains a major obstacle. Ultrasound contrast agents in the form of microbubbles have provided a unique solution to fulfill the need to shield the vectors from the host immune system and the need for site specific targeted therapy. Since the discovery of the biophysical and biological effects of microbubble sonification, multiple developments have been made to enhance its applicability in targeted drug delivery. The concurrent development of viral vectors and recent research on dual vector strategies have shown promising results. This review will explore the mechanisms and recent advancements in the knowledge of ultrasound-mediated microbubbles in targeting gene and drug therapy.

Development and application of ultrasound contrast agents in biomedicine

With the rapid development of molecular imaging, ultrasound (US) medicine has evolved from traditional imaging diagnosis to integrated diagnosis and treatment at the molecular level. Ultrasound contrast agents (UCAs) play a crucial role in the integration of US diagnosis and treatment. As the micro-bubbles (MBs) in UCAs can enhance the cavitation effect and promote the biological effect of US, UCAs have also been studied in the fields of US thrombolysis, mediated gene transfer, drug delivery, and high intensity focused US. The application range of UCAs is expanding, and the value of their applications is improving. This paper reviews the development and application of UCAs in biomedicine in recent years, and the existing problems and prospects are pointed out.

Air-Filled Microbubbles Based on Albumin Functionalized with Gold Nanocages and Zinc Phthalocyanine for Multimodal Imaging

Microbubbles are intravascular contrast agents clinically used in diagnostic sonography, echocardiography, and radiology imaging applications. However, up to date, the idea of creating microbubbles with multiple functionalities (e.g., multimodal imaging, photodynamic therapy) remained a challenge. One possible solution is the modification of bubble shells by introducing specific compounds responsible for such functions. In the present work, air-core microbubbles with the shell consisting of bovine serum albumin, albumin-coated gold nanocages, and zinc phthalocyanine were prepared using the sonication method. Various physicochemical parameters such as stability over time, size, and concentration were investigated to prove the potential use of these microbubbles as contrast agents. This work shows that hybrid microbubbles have all the necessary properties for multimodal imaging (ultrasound, raster-scanning microscopy, and fluorescence tomography), which demonstrate superior characteristics for potential theranostic and related biomedical applications.

Multivariable Dependence of Acoustic Contrast of Fluorocarbon and Xenon Microbubbles under Flow

Microbubbles (MBs) are 1 to 10 µm gas particles stabilized by an amphiphilic shell capable of responding to biomedical ultrasound with strong acoustic signals, allowing them to be commonly used in ultrasound imaging and therapy. The composition of both the shell and the core determines their stability and acoustic properties. While there has been extensive characterization of the dissolution, oscillation, cavitation, collapse and therefore, ultrasound contrast of MBs under static conditions, few reports have examined such behavior under hydrodynamic flow. In this study, we evaluate the interplay of ultrasound parameters (five different mechanical indices [MIs]), MB shell parameter (shell stiffness), type of gas (perfluorocarbon for diagnostic imaging and xenon as a therapeutic gas), and a flow parameter (flow rate) on the ultrasound signal of phospholipid-stabilized MBs flowing through a latex tube embedded in a tissue-mimicking phantom. We find that the contrast gradient (CG), a metric of the rate of decay of contrast along the length of the tube, and the contrast peak (CP), the location where the maximum contrast is reached, depend on the conditions of flow, imaging, and MB material. For instance, while the contrast near the flow inlet of the field of view is highest for a softer shell (dipalmitoylphosphatidylcholine [DPPC], C16) than for stiffer shells (distearoylphosphatidylcholine [DSPC], C18, and dibehenoylphosphatidylcholine [DBPC], C22), the contrast decay is also faster; stiffer shells provide more resistance and hence lead to slower MB dissolution/destruction. At higher flow rates, the CG is low for a fixed length of time because each MB is exposed to ultrasound for a shorter period. The CG becomes high for low flow rates, especially at high incident pressures (high MI), causing more MB destruction closer to the inlet of the field of view. Also, the CP shifts toward the inlet at low flow rates, high MIs, and low shell stiffness. We also report the first demonstration of sustained ultrasound flow imaging of a water-soluble, therapeutic gas MB (xenon). We find that an increased MB concentration is necessary for obtaining the same signal magnitude for xenon MBs. In summary, this study builds a framework depicting how multiple variables simultaneously affect the evolution of MB ultrasound contrast under flow. Depending on the MB composition, imaging conditions, transducer positioning, and image processing, building on such a framework could potentially allow for extraction of additional diagnostic information than is commonly analyzed for physiological flow.

Magneto-sonothrombolysis with combination of magnetic microbubbles and nanodroplets

This paper reports a novel technique using the rotational magnetic field oscillation and low-intensity sub-megahertz ultrasound stimulation of magnetic microbubbles (MMBs) to promote the nanodroplets (NDs) phase transition and improve the permeation of NDs into the blood clot fibrin network to enhance the sonothrombolysis efficiency. In this study, the influence of different treatment methods with a combination of MMBs and NDs on the thrombolysis rate of both unretracted and retracted clots were investigated, including the stable and inertial cavitation, tPA effects, MMBs/NDs concentration ratio, sonication factors (input voltage, duty cycle) and rotational magnetic field factors (flux density, frequency). We demonstrated that tPA-mediated magneto-sonothrombolysis in combining NDs with MMBs could significantly enhance in vitro lysis of both unretracted clots (85 ± 8.3%) and retracted clots (57 ± 6.5%) in a flow model with 30 min treatment. The results showed that the combination of MMBs and NDs substantially improves in vitro lysis of blood clots with an unprecedented lysis rate.

Ultrasound-assisted investigation of photon triggered vaporization of poly(vinylalcohol) phase-change nanodroplets: A preliminary concept study with dosimetry perspective

PURPOSE

We investigate the vaporization of phase-change ultrasound contrast agents using photon radiation for dosimetry perspectives in radiotherapy.

METHODS

We studied superheated perfluorobutane nanodroplets with a crosslinked poly(vinylalcohol) shell. The nanodroplets' physico-chemical properties, and their acoustic transition have been assessed firstly. Then, poly(vinylalcohol)-perfluorobutane nanodroplets were dispersed in poly(acrylamide) hydrogel phantoms and exposed to a photon beam. We addressed the effect of several parameters influencing the nanodroplets radiation sensitivity (energy/delivered dose/dose rate/temperature). The nanodroplets-vaporization post-photon exposure was evaluated using ultrasound imaging at a low mechanical index.

RESULTS

Poly(vinylalcohol)-perfluorobutane nanodroplets show a good colloidal stability over four weeks and remain highly stable at temperatures up to 78 °C. Nanodroplets acoustically-triggered phase transition leads to microbubbles with diameters <10 μm and an activation threshold of mechanical index = 0.4, at 7.5 MHz. A small number of vaporization events occur post-photon exposure (6MV/15MV), at doses between 2 and 10 Gy, leading to ultrasound contrast increase up to 60% at RT. The nanodroplets become efficiently sensitive to photons when heated to a temperature of 65 °C (while remaining below the superheat limit temperature) during irradiation.

CONCLUSIONS

Nanodroplets' core is linked to the degree of superheat in the metastable state and plays a critical role in determining nanodroplet' stability and sensitivity to ionizing radiation, requiring higher or lower linear energy transfer vaporization thresholds. While poly(vinylalcohol)-perfluorobutane nanodroplets could be slightly activated by photons at ambient conditions, a good balance between the degree of superheat and stability will aim at optimizing the design of nanodroplets to reach high sensitivity to photons at physiological conditions.

Contrast enhanced ultrasound in diagnosing liver lesion that spontaneously disappeared: A case report

BACKGROUND

Focal liver lesions (FLLs) are abnormal masses that are distinguishable from the surrounding liver parenchyma, solid or cystic and may be benign or malignant. They are usually detected incidentally on abdominal examinations. The classification of FLLs is very important as it directly determines the diagnosis and treatment of patients.

CASE SUMMARY

A 46-year-old male patient was admitted into the hospital with tarry stool, during the investigation of this issue an incidental FLL was detected. Upon further investigation of this “incidentaloma” computerized tomography and magnetic resonance imaging reached contradictory conclusions. The lesion was then further investigated using contrast-enhanced ultrasound (CEUS) with an initial diagnosis of idiopathic FLL was acquired and observation of the FLL over time need for final diagnosis, however in the follow up the FLL disappeared spontaneously.

CONCLUSION

CEUSs value for characterization of FLLs is undeniable, especially when other methods produce inconsistent results, is undeniable but with its limitations. Why and how the FLL disappeared is not known, and can be only hypothesized it was a pseudolesion.

Tailor-Made Single-Core PLGA Microbubbles as Acoustic Cavitation Enhancers for Therapeutic Applications

Microbubbles (MBs), being gas bubbles encapsulated inside a solid shell, have been investigated extensively in the field of therapeutic ultrasound as acoustic cavitation enhancers. Hard-shell MBs have an advantage over soft-shell MBs due to their improved stability. Poly(lactic-co-glycolic acid) (PLGA) is one of the most attractive polymers for hard-shell MB synthesis; however, very little is known regarding the effect of synthesis parameters on the acoustic cavitation activity of PLGA MBs and the tunability of this activity. In this study, by manipulating the synthesis parameters, we were able to control the characteristics of the MBs, such as their internal structure, gas core, size distribution, and shell thickness, which significantly affect the total acoustic cavitation activity that they exhibit (i.e., their cavitation dose). We showed that single-core MBs filled with C₃F₈ gas can produce cavitation effects for extended periods under continuous circulation. These MBs exhibited high stability, and their cavitation activity was not affected by prior circulation in the system. Preliminary in vivo results demonstrated that intravenously injected MBs did not cause adverse effects and produced cavitation activity that increased the permeability of the pig blood-brain barrier. Although more tests should be performed to evaluate the MB long-term safety and activity in vivo, these encouraging results suggest that our PLGA MBs have potential for future therapeutic applications as cavitation enhancers.

Evaluation of Silibinin-Loaded Microbubbles Combined with Ultrasound in Ovarian Cancer Cells: Cytotoxicity and Mechanisms

BACKGROUND

The anticancer activity of silibinin (SB) has been demonstrated in various cancer cell types. However, its low solubility and poor bioavailability limit its clinical potential in biomedical applications. Microbubbles in combination with ultrasound are promising vehicles for local drug delivery.

OBJECTIVE

The present study determined the antitumour effects and molecular mechanism of silibinin-loaded microbubbles (SBMBs) in combination with ultrasound on ovarian cancer in vitro.

METHODS

SBMBs were prepared using mechanical vibration. The viability of A2780 cells was determined using the MTT assay. Flow cytometry was performed to detect cell apoptosis and the cell cycle. The expression of receptor tyrosine kinase (RTK)-associated downstream proteins was detected using multiplex assays and Western blots.

RESULTS

The present study designed and synthesized SBMBs. SBMBs in combination with ultrasound decreased A2780 cell viability in a dose- and time-dependent manner. The half maximal inhibitory concentration (IC50) showed that the cytotoxicity of the SBMBs was approximately 1.5 times greater than that of the SB in A2780 cells. SBMBs in combination with ultrasound resulted in significantly higher apoptosis efficiency compared to the SB group, and the SBMB population of cells was arrested in the G1/G0 phase. Further experiments demonstrated that SBMBs decreased the expression of signal transducer and activator of transcription 3 (STAT3), Ak strain transforming (AKT), and extracellular signal-regulated kinase (Erk) and had a greater effect than SB in A2780 cells. Inhibitors of AKT, Erk and STAT3 promoted the cytotoxicity of SBMBs.

CONCLUSION

SBMBs in combination with ultrasound may enhance the cytotoxicity efficiency of SB via the promotion of apoptosis and cell cycle arrest in ovarian cancer cells and the inactivation of the STAT3, AKT and Erk signalling pathways.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Ultrasound-Stimulated PVA Microbubbles for Adhesive Removal from Cellulose-Based Materials: A Groundbreaking Low-Impact Methodology

In this work, we shed new light on ultrasound contrast agents applied to the field of cultural heritage as an invaluable fine-tune cleaning tool for paper artworks. In this context, one of the primary and challenging issues is the removal of modern adhesives from paper artifacts. Modern adhesives are synthetic polymers whose presence enhances paper degradation and worsens its optical features. A thorough analytical and high-spatial-resolution combined study was successfully performed to test the capability of poly(vinyl alcohol)-based microbubbles stimulated by a proper noninvasive 1 MHz ultrasound field exposure in removing these adhesives from paper surfaces, in the absence of volatile invasive and toxic chemicals and without damaging paper and/or leaving residues. We demonstrate that poly(vinyl alcohol)-shelled microbubbles are suitable for interacting with paper surfaces, targeting and boosting in a few minutes the nondamaging removal of adhesive particles from paper samples thanks to their peculiar shell composition together with their ultrasound dynamics.

Endoglin/CD105-Based Imaging of Cancer and Cardiovascular Diseases: A Systematic Review

Molecular imaging of pathologic lesions can improve efficient detection of cancer and cardiovascular diseases. A shared pathophysiological feature is angiogenesis, the formation of new blood vessels. Endoglin (CD105) is a coreceptor for ligands of the Transforming Growth Factor-β (TGF-β) family and is highly expressed on angiogenic endothelial cells. Therefore, endoglin-based imaging has been explored to visualize lesions of the aforementioned diseases. This systematic review highlights the progress in endoglin-based imaging of cancer, atherosclerosis, myocardial infarction, and aortic aneurysm, focusing on positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), near-infrared fluorescence (NIRF) imaging, and ultrasound imaging. PubMed was searched combining the following subjects and their respective synonyms or relevant subterms: "Endoglin", "Imaging/Image-guided surgery". In total, 59 papers were found eligible to be included: 58 reporting about preclinical animal or in vitro models and one ex vivo study in human organs. In addition to exact data extraction of imaging modality type, tumor or cardiovascular disease model, and tracer (class), outcomes were described via a narrative synthesis. Collectively, the data identify endoglin as a suitable target for intraoperative and diagnostic imaging of the neovasculature in tumors, whereas for cardiovascular diseases, the evidence remains scarce but promising.

Three-dimensional visualization and improved quantification with super-resolution ultrasound imaging - validation framework for analysis of microvascular morphology using a chicken embryo model

The purpose of this study was to improve the morphological analysis of microvascular networks depicted in three-dimensional (3D) super-resolution ultrasound (SR-US) images. This was supported by qualitative and quantitative validation by comparison to matched brightfield microscopy and traditional B-mode ultrasound (US) images. Contrast-enhanced US (CEUS) images were collected using a preclinical US scanner (Vevo 3100, FUJIFILM VisualSonics Inc.) equipped with an MX250 linear array transducer. CEUS imaging was performed after administration of a microbubble (MB) contrast agent into the vitelline network of a developing chicken embryo. Volume data was collected by mechanically scanning the US transducer throughout a tissue volume-of-interest in 90μm step increments. CEUS images were collected at each increment and stored as in-phase/quadrature data (2000 frames at 152 frames per sec). SR-US images were created for each cross-sectional plane using established data processing methods. All SR-US images were then used to reconstruct a final 3D volume for vessel diameter (VD) quantification and for surface rendering. VD quantification from the 3D SR-US data exhibited an average error of 6.1% ± 6.0% when compared with matched brightfield microscopy images, whereas measurements from B-mode US images had an average error of 77.1% ± 68.9%. Volume and surface renderings in 3D space enabled qualitative validation and improved visualization of small vessels below the axial resolution of the US system. Overall, 3D SR-US image reconstructions depicted the microvascular network of the developing chicken embryos. Improved visualization of isolated vessels and quantification of microvascular morphology from SR-US images achieved a considerably greater accuracy compared to B-mode US measurements.

Challenges and strategies for the delivery of biologics to the cornea

Biologics, like peptides, proteins and nucleic acids, have proven to be promising drugs for the treatment of numerous diseases. However, besides the off label use of the monoclonal antibody bevacizumab for the treatment of corneal neovascularization, to date no other biologics for corneal diseases have reached the market. Indeed, delivering biologics in the eye remains a challenge, especially at the level of the cornea. While it appears to be a rather accessible tissue for the administration of drugs, the cornea in fact presents several anatomical barriers to delivery. In addition, also intracellular delivery barriers need to be overcome to achieve a promising therapeutic outcome with biologics. This review outlines efforts that have been reported to successfully deliver biologics into the cornea. Biochemical and physical methods for achieving delivery of biologics in the cornea are discussed, with a critical view on their efficacy in overcoming corneal barriers.

Contrast-enhanced US for the Interventional Radiologist: Current and Emerging Applications

US is a powerful and nearly ubiquitous tool in the practice of interventional radiology. Use of contrast-enhanced US (CEUS) has gained traction in diagnostic imaging given the recent approval by the U.S. Food and Drug Administration (FDA) of microbubble contrast agents for use in the liver, such as sulfur hexafluoride lipid-type A microspheres. Adoption of CEUS by interventional radiologists can enhance not only procedure guidance but also preprocedure patient evaluation and assessment of treatment response across a wide spectrum of oncologic, vascular, and nonvascular procedures. In addition, the unique physical properties of microbubble contrast agents make them amenable as therapeutic vehicles in themselves, which can lay a foundation for future therapeutic innovations in the field in drug delivery, thrombolysis, and vascular flow augmentation. The purpose of this article is to provide an introduction to and overview of CEUS aimed at the interventional radiologist, highlighting its role before, during, and after frequently practiced oncologic and vascular interventions such as biopsy, ablation, transarterial chemoembolization, detection and control of hemorrhage, evaluation of transjugular intrahepatic portosystemic shunts (TIPS), detection of aortic endograft endoleak, thrombus detection and evaluation, evaluation of vascular malformations, lymphangiography, and percutaneous drain placement. Basic physical principles of CEUS, injection and scanning protocols, and logistics for practice implementation are also discussed. Early adoption of CEUS by the interventional radiology community will ensure rapid innovation of the field and development of future novel procedures. Online supplemental material is available for this article. ^©RSNA, 2020.

A Robust Oxygen Microbubble Radiosensitizer for Iodine-125 Brachytherapy

Iodine-125 (125I) brachytherapy, a promising form of radiotherapy, is increasingly applied in the clinical treatment of a wide range of solid tumors. However, the extremely hypoxic microenvironment in solid tumors can cause hypoxia-induced radioresistance to 125I brachytherapy, resulting in therapeutic inefficacy. In this study, the aim is to sensitize hypoxic areas in solid tumors using ultrasound-activated oxygen microbubbles for 125I brachytherapy. A modified emulsion freeze-drying method is developed to prepare microbubbles that can be lyophilized for storage and easily reconstituted in situ before administration. The filling gas of the microbubbles is modified by the addition of sulfur hexafluoride to oxygen such that the obtained O2/SF6 microbubbles (OS MBs) achieve a much longer half-life (>3×) than that of oxygen microbubbles. The OS MBs are tested in nasopharyngeal carcinoma (CNE2) tumor-bearing mice and oxygen delivery by the OS MBs induced by ultrasound irradiation relieve hypoxia instantly. The post-treatment results of brachytherapy combined with the ultrasound-triggered OS MBs show a greatly improved therapeutic efficacy compared with brachytherapy alone, illustrating ultrasound-mediated oxygen delivery with the developed OS MBs as a promising strategy to improve the therapeutic outcome of 125I brachytherapy in hypoxic tumors.

Multifocused Ultrasound Therapy for Controlled Microvascular Permeabilization and Improved Drug Delivery

Focused ultrasound (FUS) exposure of micro-bubble (MB) contrast agents can transiently increase microvascular permeability allowing anticancer drugs to extravasate into a targeted tumor tissue. Either fixed or mechanically steered in space, most studies to date have used a single element focused transducer to deliver the ultrasound (US) energy. The goal of this study was to investigate various multi-FUS strategies implemented on a programmable US scanner (Vantage 256, Verasonics Inc.) equipped with a linear array for image guidance and a 128-element therapy transducer (HIFUPlex-06, Sonic Concepts). The multi-FUS strategies include multi-FUS with sequential excitation (multi-FUS-SE) and multi-FUS with temporal sequential excitation (multi-FUS-TSE) and were compared to single-FUS and sham treatment. This study was performed using athymic mice implanted with breast cancer cells ( N = 20 ). FUS therapy experiments were performed for 10 min after a solution containing MBs (Definity, Lantheus Medical Imaging Inc.) and near-infrared (NIR, surrogate drug) dye were injected via the tail vein. The fluorescent signal was monitored using an in vivo optical imaging system (Pearl Trilogy, LI-COR) to quantify intratumoral dye accumulation at baseline and again at 0.1, 24, and 48 h after receiving US therapy. Animals were then euthanized for ex vivo dye extraction analysis. At 48 h, fluorescent tracer accumulation within the tumor space for the multi-FUS-TSE therapy group animals was found to be 67.3%, 50.3%, and 36.2% higher when compared to sham, single-FUS, and multi-FUS-SE therapy group measures, respectively. Also, dye extraction and fluorescence measurements from excised tumor tissue found increases of 243.2%, 163.1%, and 68.1% for the multi-FUS-TSE group compared to sham, single-FUS, and multi-FUS-SE therapy group measures, respectively. In summary, experimental results revealed that for a multi-FUS sequence, increased microvascular permeability was considerably influenced by both the spatial and temporal aspects of the applied US therapy.

Nonlinear dynamics and bifurcation structure of ultrasonically excited lipid coated microbubbles

In many applications, microbubbles (MBs) are encapsulated by a lipid coating to increase their stability. However, the complex behavior of the lipid coating including buckling and rupture sophisticates the dynamics of the MBs and as a result the dynamics of the lipid coated MBs (LCMBs) are not well understood. Here, we investigate the nonlinear behavior of the LCMBs by analyzing their bifurcation structure as a function of acoustic pressure. We show that, the LC can enhance the generation of period 2 (P2), P3, higher order subharmonics (SH), superharmonics and chaos at very low excitation pressures (e.g. 1 kPa). For LCMBs sonicated by their SH resonance frequency and in line with experimental observations with increasing pressure, P2 oscillations exhibit three stages: generation at low acoustic pressures, disappearance and re-generation. Within non-destructive oscillation regimes and by pressure amplitude increase, LCMBs can also exhibit two saddle node (SN) bifurcations resulting in possible abrupt enhancement of the scattered pressure. The first SN resembles the pressure dependent resonance phenomenon in uncoated MBs and the second SN resembles the pressure dependent SH resonance. Depending on the initial surface tension of the LCMBs, the nonlinear behavior may also be suppressed for a wide range of excitation pressures.

Nanoplatforms for Targeted Stimuli-Responsive Drug Delivery: A Review of Platform Materials and Stimuli-Responsive Release and Targeting Mechanisms

To achieve the promise of stimuli-responsive drug delivery systems for the treatment of cancer, they should (1) avoid premature clearance; (2) accumulate in tumors and undergo endocytosis by cancer cells; and (3) exhibit appropriate stimuli-responsive release of the payload. It is challenging to address all of these requirements simultaneously. However, the numerous proof-of-concept studies addressing one or more of these requirements reported every year have dramatically expanded the toolbox available for the design of drug delivery systems. This review highlights recent advances in the targeting and stimuli-responsiveness of drug delivery systems. It begins with a discussion of nanocarrier types and an overview of the factors influencing nanocarrier biodistribution. On-demand release strategies and their application to each type of nanocarrier are reviewed, including both endogenous and exogenous stimuli. Recent developments in stimuli-responsive targeting strategies are also discussed. The remaining challenges and prospective solutions in the field are discussed throughout the review, which is intended to assist researchers in overcoming interdisciplinary knowledge barriers and increase the speed of development. This review presents a nanocarrier-based drug delivery systems toolbox that enables the application of techniques across platforms and inspires researchers with interdisciplinary information to boost the development of multifunctional therapeutic nanoplatforms for cancer therapy.

Iridium(III) Complex-Loaded Perfluoropropane Nanobubbles for Enhanced Sonodynamic Therapy

Sonodynamic therapy (SDT) is a novel promising approach for the minimally invasive treatment of cancer derived from photodynamic therapy (PDT). In this study, we have explored an effective sonosensitizer for SDT by loading the iridium(III) complex [Ir(ppy)₂(en)] OOCCH₃, where ppy = 2-phenylpyridine and en = ethylenediamine], from now on referred to as Ir, with high photosensitizing ability, into echogenic nanobubbles (Ir-NBs). Akin to photosensitizers, sonosensitizers are acoustically activated by deep-tissue-penetrating low-frequency ultrasound (US) resulting in a localized therapeutic effect attributed to an excessive generation of reactive oxygen species (ROS). The Ir-NB formulation was optimized, and the in vitro characterizations were carried out, including physical properties, acoustic performance, intracellular ROS generation, and cytotoxicity against two human cancer cell lines. Ir-NBs had an average size of 303.3 ± 91.7 nm with a bubble concentration of 9.28 × 10¹⁰ particles/mL immediately following production. We found that the initial Ir feeding concentration had a negligible effect on the NB size, but affected the bubble concentration as well as the acoustic performance of the NBs. Through a combination of sonication and Ir-NBs treatment, an increase of 68.8% and 69.6% cytotoxicity in human ovarian cancer cells (OVCAR-3) and human breast cancer cells (MCF-7), respectively, was observed compared to the application of Ir-NBs alone. Furthermore, Ir-NBs exposed to the US also induced the highest levels of intracellular ROS generation compared to free Ir and free Ir with empty NBs. The combination of these results suggests that the differences in treatment efficacy is a direct result of acoustic cavitation. These results provide evidence that US activated Ir-loaded NBs have the potential to become an effective sonosensitizer for SDT.

Externally triggered release of growth factors - A tissue regeneration approach

Tissue regeneration aims to achieve functional restoration following injury by creating an environment to enable the body to self-repair. Strategies for regeneration rely on the introduction of biomaterial scaffolding, cells and bioactive molecules into the body, at or near the injury site. Of these bioactive molecules, growth factors (GFs) play a pivotal role in directing regenerative pathways for many cell populations. However, the therapeutic use of GFs has been limited by the complexity of biological injury and repair, and the properties of the GFs themselves, including their short half-life, poor tissue penetration, and off-target side effects. Externally triggered delivery systems have the potential to facilitate the delivery of GFs into the target tissues with considerations of the timing, sequence, amount, and location of GF presentation. This review briefly discusses the challenges facing the therapeutic use of GFs, then, we discuss approaches to externally trigger GF release from delivery systems categorised by stimulation type; ultrasound, temperature, light, magnetic fields and electric fields. Overall, while the use of GFs for tissue regeneration is still in its infancy, externally controlled GF delivery technologies have the potential to achieve robust and effective solutions to present GFs to injured tissues. Future technological developments must occur in conjunction with a comprehensive understanding of the biology at the injury site to ensure translation of promising technologies into real world benefit.

Microbubbles Containing Lysolipid Enhance Ultrasound-Mediated Blood-Brain Barrier Breakdown In Vivo

Ultrasound and microbubbles (MBs) offer a noninvasive method of temporarily enhancing blood-brain barrier (BBB) permeability to therapeutics. To reduce off-target effects, it is desirable to minimize the ultrasound pressures required. It has been shown that a new formulation of MBs containing lysolipids (Lyso-MBs) can increase the cellular uptake of a model drug in vitro. The aim of this study is to investigate whether Lyso-MBs can also enhance BBB permeability in vivo. Female BALB/c mice are injected with either Lyso-MBs or control MBs and gadolinium-DTPA (Gd-DTPA) and exposed to ultrasound (500 kHz, 1 Hz pulse repetition frequency, 1 ms pulse length, peak-negative pressures 160-480 kPa) for 2 min. BBB permeabilization is measured via magnetic resonance imaging (7.0 T) of Gd-DTPA extravasation and subsequent histological examination of brain tissue to assess serum immunoglobulin G (IgG) extravasation (n = 8 per group). An approximately twofold enhancement in BBB permeability is produced by the Lyso-MBs at the highest ultrasound pressure compared with the control. These findings indicate that modifying the composition of phospholipid-shelled MBs has the potential to improve the efficiency of BBB opening, without increasing the ultrasound pressure amplitude required. This is particularly relevant for delivery of therapeutics deep within the brain.

Ultrasound mediated delivery of quantum dots from a proof of concept capsule endoscope to the gastrointestinal wall

Biologic drugs, defined as therapeutic agents produced from or containing components of a living organism, are of growing importance to the pharmaceutical industry. Though oral delivery of medicine is convenient, biologics require invasive injections because of their poor bioavailability via oral routes. Delivery of biologics to the small intestine using electronic delivery with devices that are similar to capsule endoscopes is a promising means of overcoming this limitation and does not require reformulation of the therapeutic agent. The efficacy of such capsule devices for drug delivery could be further improved by increasing the permeability of the intestinal tract lining with an integrated ultrasound transducer to increase uptake. This paper describes a novel proof of concept capsule device capable of electronic application of focused ultrasound and delivery of therapeutic agents. Fluorescent markers, which were chosen as a model drug, were used to demonstrate in vivo delivery in the porcine small intestine with this capsule. We show that the fluorescent markers can penetrate the mucus layer of the small intestine at low acoustic powers when combining microbubbles with focused ultrasound during in vivo experiments using porcine models. This study illustrates how such a device could be potentially used for gastrointestinal drug delivery and the challenges to be overcome before focused ultrasound and microbubbles could be used with this device for the oral delivery of biologic therapeutics.